The present invention relates to electrical leads and more particularly to electrical leads for devices immersed in a cryogenic fluid.
When electrical current is passed along a conductor to an electrical device immersed in a cryogenic fluid, such as for example liquid helium contained in a Dewar, heat conduction from the room temperature enviroment and the ohmic heat generated in the conductor results in the introduction of heat into the cryogenic liquid. Likewise heat from the warm area outside of the Dewar is passed along by the conductor and introduced to the cryogenic liquid. It is of course desirable to reduce this heat introduced to the cryogenic liquid to a minimum in order to reduce the loss of liquid due to boil-off.
Attempts have been made in the past to reduce the quantity of heat reaching the cryogenic liquid by cooling the electrical leads to the device immersed in the liquid by utilizing the boil-off gas from the liquid. The boil-off gas leaves the Dewar by means of a duct which has contained therein an electrical conductor in such a manner as to be in heat-exchange relationship with the gas flowing therethrough. One embodiment utilized a mesh screen rolled into a cylinder and suspended within the duct to act as the lead. The strands of the mesh screen extending in the longitudinal direction carried the current, while the transverse strands functioned as cooling members. A large number of small conductors maximized the effective surface area exposed to and in contact with the boil-off gas coolant. The disadvantage of such an arrangement was that the length of the duct must be at least as long as the lead, which in some applications was cumbersome.
In order to reduce the length of the duct to something less than the entire lead length, the lead was formed of an electrically conducting strip edge-wound to form a helix that when installed in a cylinder defined a helical duct through which the boil-off gas coolant flowed in heat-exchange relationship with the conductor strip. In order to prevent inefficient laminar flow, increased thermal conductivity was obtained when the gas flow was caused to be turbulent by using a crinkled copper strip for the conductor. The thickness of strip that can accommodate such crinkling will limit the current carrying capacity of the conductor.